Plant growth regulators are chemicals used in some manner to alter the growth of plants, blossoms, or fruits. Hormones are the natural plant growth regulators produced by plants that, among other things, control growth, initiate flowering, cause blossoms to fall, cause fruit and leaves to fall, control initiation and termination of dormancy, and stimulate root development. Agricultural success depends, in large part, on external plant growth regulation to adapt the natural growth patterns of plants to the needs of the agricultural community.
The use and development of plant growth regulators began with the discovery that acetylene and ethylene would promote flowering in pineapples. This occurred in 1932, and by 1934, auxins were discovered to enhance root formation in cuttings. During the last 60 years, major discoveries in the area of plant growth regulators have resulted in the development of seedless fruits; the prevention of early, premature drop of fruits; the promotion of heavy setting of fruit blossoms; the prevention of sprouting in stored potatoes and onions; and the inhibition of buds in nursery stock and fruit trees to prolong dormancy.
The American Society for Horticultural Science recognizes six classes of plant growth regulators: auxins, gibberellins, cytokinins, ethylene generators, inhibitors, and growth retardants. The last class of regulator, growth retardants, includes an assorted group of substances that inhibit or retard certain physiological processes in plants. Two types of retardants have been developed for use: hormones and other substances that occur naturally in plants, and recently discovered, synthetic compounds. Examples of naturally occurring inhibitors in plants are benzoic acid, gallic acid, cynnamic acid, and (s)-abscisic acid. (s)-Abscisic acid, the most recently discovered naturally occurring hormone in plants, regulates gas exchange and water loss from plants.
Synthetic inhibitors include mepiquat chloride (Pix.RTM.), chlormequat chloride (Cycocel.RTM.), ancymidol (A-Rest.RTM.), dikegulac sodium (Atrinal.RTM.), daminozide (B-Nine.RTM.), and mefluidide (Embark.RTM.). Many of these synthetic growth inhibitors inhibit the synthesis of gibberellins, which results in the slowing or halting of elongations of leaves and stems, both in grasses and broad leaf plants.
A variety of synthetic plant growth retardants, such as Alar.RTM., Bonzi.RTM., and A-Rest.RTM. have been used previously in commercial settings to prevent plants from becoming overgrown. These chemicals are toxic to humans, however, at relatively low concentrations, and some are suspected carcinogens. Consequently, these retardants are not labelled for commercial application on food-producing plants and are under scrutiny in the ornamented plant industry.
The use of plant growth retardants and flower initiators in the commercial plant-growing industry is critical to the economic well-being of the industry. However, the use of such agents by the industry is limited, since many synthetic chemicals are either no longer in use due to their suspected carcinogenic effect or toxicity, or because they are not approved for certain types of plants. Naturally occurring hormones in plants, moreover, cannot be harvested economically and are too expensive for large scale use. The impact of such restrictions is that entire crops may be lost due to seasonal variations, for example, rain, that delays planting. Thus, a plant growth regulator is greatly needed by the industry that is safe, available in large quantities, and economical to use.